It is the action of raising and adding more chips than the bet of the previous player. Please note that there is a minimum amount to be observed when raising.

The minimum amount of raises
In order to avoid long betting rounds, raises must be made according to the rule that the amount of the raise must be greater than or equal to twice the amount of the Big Blind.
For example, if the Big Blind amount is 20, the first raise must be at least 40. If a player decides to raise to 50, the next player to raise must bet at least 70.